Explain to me wheel offset

Ok, I've been shoping for some new wheels. I found some Mickey Thompson's that I really like but it says -12mm offset. Is that a lot? cause my ruler says it's about a half inch. ALSO what the heck is the difference between backspacing and offset? Is there a problem with offset?

backspacing is the distance between the back of the rim and the back of the hub. basically if you set your rim on the ground face up and put a tape measure through the center of the wheel to the ground that will give you your backspacing(in inches). the way offset was explained to me is the distance that your hub is from the center of the rim(in mm). you have a "+" offset(how far it is forward of the center) and a "-" offset,(how far it is backward of the center). they are 2 ways of measuring the same thing.

backspacing is the distance between the back of the rim and the back of the hub. basically if you set your rim on the ground face up and put a tape measure through the center of the wheel to the ground that will give you your backspacing(in inches). the way offset was explained to me is the distance that your hub is from the center of the rim(in mm). you have a "+" offset(how far it is forward of the center) and a "-" offset,(how far it is backward of the center). they are 2 ways of measuring the same thing.

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Ok, so are you saying the offset can affect the backspacing? cause the wheel's i'm looking at have a 4.5" backspacing and the -12mm offset.